Merlin identified the song as Swainson’s Warbler
Just the head and some loose large feathers, something appears to have taken the body this morning and left the head.
I very briefly saw the father at the nest but couldn't get a picture of him; he fled the instant I tried to get close. I think the babies are somewhat habituated to seeing humans through the window by now, but the mother much less so, and the father absolutely not at all.
I also saw one of the babies preening themselves, and I think that's what I was trying to capture in the second picture.